1. Field of the Invention
The present invention relates to signal line driving circuits for providing output signals with approximately constant output currents, and in particular, to subscriber line interface circuits for telephone lines.
2. Description of the Related Art
A subscriber line interface circuit (SLIC) is used for each pair of wires (xe2x80x9ctipxe2x80x9d and xe2x80x9cringxe2x80x9d) forming a subscriber telephone line and is responsible for conveying both incoming and outgoing signals (e.g., voice, facsimile and data) while providing necessary power and impedance matching. Such circuit is typically located in the central office and is analog in its function, although digital versions (e.g., ISDN) are being used more often.
When the subscriber telephone is on-hook the DC output current, neglecting any leakage impedances, is substantially zero. When the subscriber loop goes into an off-hook state, industry standards require a DC output current through the subscriber loop to be approximately constant, e.g., within the nominal range of 30-40 milliamperes (mA). This DC current provides power to the subscriber telephone circuitry, such as the digital keypad. The impedance of the subscriber loop will depend upon the particular telephone, or telephones, connected to the loop, as well as the transmission length of the loop itself. Therefore, the SLIC must be designed to provide the nominal required DC output current for this range of loop impedances while still providing for transmission of the AC signals (voice, facsimile, etc.).
Based upon the foregoing, the following typical scenarios will be encountered. With an output current of 30 mA, a nominal DC power supply voltage of 50 volts for the SLIC, and a subscriber loop impedance of 1000 ohms (resistive), the output voltage presented to the subscriber loop will be 30 volts (=30 mAxc3x971000 ohms). Accordingly, 20 volts will be dropped across the SLIC output circuitry, thereby resulting in approximately 0.6 watts (=30 mAxc3x9720 volts) of power dissipation in the output circuitry of the SLIC. However, if the subscriber loop impedance is instead only 500 ohms, then the output voltage becomes 15 volts (=30 mAxc3x97500 ohms). This results in 35 volts being dropped across the output circuitry of the SLIC, which, in turn, results in an internal power dissipation of approximately 1.05 watts (=30 mAxc3x9735 volts). Hence, it can be seen that, depending upon the subscriber loop impedance, a wide variance in the internal power dissipation of the SLIC can be encountered. Such a wide variance in internal power dissipation imposes substantial design constraints for the SLIC, and prevents such SLIC from performing at maximum efficiency.
A number of attempts to minimize the internal power dissipation of the SLIC have included such techniques as using an external resistor (connected in series with the power supply) for dissipating the excess power and using a switching voltage regulator. However, both techniques have significant disadvantages. Simply relocating the power dissipation to an external resistor does not improve overall efficiency of the system, and while a switching voltage regulator may improve power efficiency, significant switching noise can be induced into the subscriber loop.
Accordingly, it would be desirable to have a technique by which internal power dissipation can be automatically reduced by maximizing power efficiency and avoiding any introduction of signal noise.
A signal line driving circuit with self-controlled internal power dissipation in accordance with the present invention minimizes internal power dissipation while maximizing overall power efficiency and avoiding introduction of extraneous signal noise into the system.
In accordance with one embodiment of the present invention, a signal line driving circuit with power control for selectively reducing internal power dissipation when driving an external load includes a signal driver circuit and a power control circuit. The signal driver circuit is configured to connect and provide an output signal to an external impedance and to receive a source current and an input signal which corresponds to such output signal and in accordance therewith provide such output signal and a control signal which varies in relation to such output signal. The output signal includes an output current which is approximately constant and an output voltage which varies in relation to the external impedance and output current. The power control circuit, coupled to the signal driver circuit, is configured to connect to a plurality of voltage sources and receive therefrom a plurality of source voltages and to receive the control signal and in accordance therewith convey the source current from one of the plurality of voltage sources to the signal driver circuit.
In accordance with another embodiment of the present invention, a method of driving an external load via a signal line while selectively reducing power dissipation includes the steps of:
connecting to an external impedance;
connecting to a plurality of voltage sources;
applying an output signal to the external impedance;
receiving a source current and an input signal which corresponds to the output signal and in accordance therewith generating the output signal and a control signal which varies in relation to the output signal, wherein the output signal includes an output current which is approximately constant and an output voltage which varies in relation to the external impedance and the output current;
receiving a plurality of source voltages from the voltage sources; and
receiving the control signal and in accordance therewith conveying the source current from one of the plurality of voltage sources.
These and other features and advantages of the present invention will be understood upon consideration of the following detailed description of the invention and the accompanying drawings.